Generally, in the membrane distillation, a phase change occurs on a surface of a hydrophobic polymer membrane, and steam passes through fine pores in the surface of the membrane and then is concentrated and separated.
In addition, the membrane distillation may be used in a desalinization process for separating and removing non-volatile substances or low-volatile substances or used for separating organic materials with high volatility in a solution.
Since the concept of the membrane distillation was proposed in 1960s, the membrane distillation has been studied mainly in USA, Europe, Japan and Australia so far. Recently, there is an active movement for replacing the membrane distillation separation process with an existing separation process using evaporation or a reverse osmosis membrane.
At present, an evaporation method and a reverse osmosis method used for a pure water producing process or a desalination process consume much energy. In particular, the reverse osmosis method should perform several preprocesses before usage due to contamination and fouling, which causes difficulty in operation and management and consumes much electric energy as a power source of a pump to cause increased maintenance costs.
Meanwhile, the membrane distillation uses a porous membrane and operates at a lower pressure in comparison to the extracorporeal ultrafiltration method and the reverse osmosis method. The membrane distillation allows separation by means of a difference in partial vapor pressure.
In addition, if the membrane distillation is used, there is no entrainment which is present at a traditional distillation method, when separating and removing non-volatile substances such as salt, and also a filter or membrane operating at a high temperature may not be used.
Due to such advantages of the membrane distillation separation process, a desalination process using the membrane distillation is spotlighted as one of competitive methods for producing drinking water over the world due to low-cost utility and durable separation device.
Meanwhile, the membrane distillation uses a hydrophobic polymer membrane, but since a surface tension of a solvent or solute (hydrophilic material) is greater than a membrane surface, liquid does not pass through membrane pores but makes repulsion at the membrane surface. Also, the material to be separated is phase-converted into steam at an entrance of the pore in the membrane surface and then diffused and permeated into the pores to be finally concentrated and separated at treated water.
The membrane distillation is performed at a membrane module composed of an inlet water part at which an inflow solution passes through the membrane and a treated water part at which the material to be separated is concentrated and separated.
However, since the membrane distillation uses the hydrophobic polymer membrane as a membrane material, the surface of the membrane may be contaminated due to various contaminants contained in a contaminated raw water at the inlet water part of the membrane distillation, and when the membrane is partially hydrophilic-treated, the inlet water may directly pass through the hydrophilic-treated portion, which is called a membrane wetting phenomenon.
If the membrane wetting phenomenon occurs, raw water of the inlet water passes through the membrane, and thus the membrane distillation process for obtaining pure water may not be operated, and if the membrane wetting phenomenon occurs, a membrane de-wetting process for removing membrane scaling material in the pores must be performed.
In an existing technique, the membrane wetting phenomenon has been measured by means of an indirect method, for example by checking an increase of electric conductivity at the treated water part, but it is impossible to measure a location of the membrane where the membrane wetting phenomenon occurs.
Therefore, in order to check membrane scaling during the membrane distillation process, it is demanded to study a real-time direct monitoring method for checking a location of the membrane where the membrane wetting phenomenon occurs.